MPlayer is a media player capable of handling multiple multimedia file formats.
Sven Tantau discovered a heap overflow in the code handling the strf chunk of PCM audio streams.
An attacker could craft a malicious video or audio file which, when opened using MPlayer, would end up executing arbitrary code on the victim's computer with the permissions of the user running MPlayer.
You can mitigate the issue by adding "ac=-pcm," to your MPlayer configuration file (note that this will prevent you from playing uncompressed audio).
All MPlayer users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=media-video/mplayer-1.0_pre7-r1"